It is known to provide an annular nozzle or shroud around a marine propeller in order to improve the propeller efficiency. Such a nozzle may be immovably fixed to the hull of a marine vessel (see, for example, Swedish Pat. No. 98.659) or it may be pivotally connected to the hull by means of a rudder stock (see, for example, Swedish Pat. No. 137.658), whereby the nozzle also acts as a steering device. It has been found that the clearance between the propeller blade tips and the nozzle should be as small as possible to obtain the highest possible propeller efficiency and the greatest thrust from the propeller-nozzle combination.
In shrouded marine propellers having removable (i.e., detachable) blades, for example, a built-up propeller or a variable pitch propeller, it is desirable to construct the propeller and shroud in such a way that the propeller blades can be installed or removed, such as for changing the blades, without dismounting the propeller or parts of the nozzle. Such propellers are usually made so that when a propeller blade has to be replaced it must first be moved a distance radially outward from the propeller hub toward the inner wall of the nozzle. This is particularly so in cases where the propeller blade flanges, by which the propeller blades are connected to the propeller hub, have guide edges and guide pins, the flanges being recessed in propeller blade sites formed on the propeller hub. Thus, for marine vessels having immovable nozzles, the clearance between the propeller blade tips and the inner wall of the nozzle must be at least equal to the radial distance necessary for removing the propeller blades.
To provide the smallest possible clearance between the propeller blade tips and the nozzle without dismantling or moving any part of the nozzle, most of the known solutions have concentrated on reducing the radial distance necessary for removing the propeller blades. This has been accomplished by making the guide edges and guide pins between the propeller hub and the propeller blade flanges as small as possible or by providing the blade flanges and the hub with spherical portions so that the blade flanges do not have to be recessed into the hub.
Although the known solutions succeed in minimizing the necessary clearance between the propeller blade tips and the nozzle, they jeopardize the safe attachment and sealing of the propeller blades and the propeller hub. Furthermore, where the clearance between the propeller blade tips and the nozzle is minimized, there are difficulties in transporting the propeller blade in and out of the nozzle.
Although pivotable nozzles may facilitate the replacement of the propeller blades by permitting the nozzle to be pivoted about its rudder stock so that the propeller hub can be turned to a position in which the propeller blade to be replaced is horizontal, positioning the propeller blade in a horizontal orientation creates several disadvantages. For example, in controllable pitch propellers the hub must be emptied of oil before the propeller blade can be moved to a horizontal position. Also, it is considerably more difficult to remove a large or heavy propeller blade when it is in a horizontal position, as opposed to a vertical position.